A few years back I’ve constructed a few large planters to place on top of the garden tiles. However the low, wide planters turn out to be prone to drying out. A layer of larger gravel placed on top of the surface in the driest planter, aimed to countered the evaporation somewhat.
The rather soft stone sucks up the water so it becomes even softer, and turns into a useable source of loam for Trypoxylon and Auplopus carbonarius wasps. That latter species is most enthusiastic about the material. Hanging on the stones multiple specimens gnaw particles loose and collect them into small, humid balls. The moisture is collected at water sources nearby and used to make the grit manageable and transportable.
References1 Nederlands Soortenregister